I may have technique-

Absorb the Phosphoric acid in Distilled water - 50ml(Taken as Absorbing solution),

If it is acidic - neutral it using NaOH (or you amy skip this step)

Then develop the color by adding Vanado-molybdate solution (10 ml), Color will be yellow if phosphorus is present, Then after 10 min, take the absorbance at 470nm.

You offcourse have to make a calibration curve with phosphorus standard.

Dear Bruno

You can use the METHOD 0060 (DETERMINATION OF METALS IN STACK EMISSIONS) which has been developed by EPA. You can find it from bellow:

https://www.epa.gov/sites/production/files/2015-12/documents/0060.pdf

Regards,

Ata